JP2000167404A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for purification of exhaust gas which can develop and maintain excellent oxygen storage capacity(OSC) even at high temp. such as >=900 deg.C. SOLUTION: In the catalyst for purification of exhaust gas having a coating layer 3 containing catalytic active substances C(c) and rare earth oxides B(b) formed on a supporting carrier, the rare earth oxides B include at least praseodymium oxide or terbium oxide. The coating layer is preferably formed as plural layers 3A and 3B, and at least one layer 3A (3B) contains praseodymium oxide or terbium oxide. When palladium is used as the catalytically active substance C(c), palladium is incorporated into the inner layer 3A, and when palladium and rhodium are used together, these catalytically active substances C(c) are separately added to the respective layers 3A and 3B.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to nitrogen oxides (NO _x ), carbon monoxide (CO), and hydrocarbons (H) contained in exhaust gas discharged from an internal combustion engine such as an automobile.
The present invention relates to an exhaust gas purifying catalyst for efficiently purifying C) and the like.

[0002]

2. Description of the Related Art As an exhaust gas purifying catalyst, a structure in which a wash coat layer (coating layer) is formed on the inner surface of a cell of a honeycomb support made of cordierite or the like is generally employed. The washcoat layer is made of a catalytically active substance such as platinum, palladium or rhodium, or a highly heat-resistant inorganic oxide such as alumina. Further, the washcoat layer contains cerium oxide having an oxygen storage capacity (hereinafter, referred to as “OSC”), thereby improving the activity of the exhaust gas purifying catalyst.

[0003] However, cerium oxide is, for example, 90
At a high temperature of 0 ° C. or higher, grain growth (sintering) occurs, the specific surface area decreases, and the OSC decreases. In this case, since it is not possible to sufficiently maintain the activity of the exhaust gas purifying catalyst in a high-temperature atmosphere, it is considered that zirconium, yttrium, or the like is combined with cerium oxide to suppress sintering of cerium oxide.

In this case, if the OSC per unit weight of the composite oxide after exposure to a high-temperature atmosphere is small,
In order to maintain the activity of the exhaust gas purifying catalyst as desired over a long period of time, a relatively large amount of a complex oxide (OSC material) must be used. In this case, the thickness of the washcoat layer increases, and the exhaust resistance of the engine deteriorates. In order to avoid such a problem, it is necessary to develop an OSC material having a large OSC per unit weight even after exposure to a high-temperature atmosphere.

The present invention has been conceived under the circumstances described above, and maintains excellent oxygen storage capacity (OSC) even after endurance in a high-temperature atmosphere of 900 ° C. or more.
An object of the present invention is to provide an exhaust gas purifying catalyst that can exhibit its effects.

[0006]

DISCLOSURE OF THE INVENTION In order to solve the above-mentioned problems, the present invention takes the following technical means.

That is, the exhaust gas purifying catalyst provided by the present invention is an exhaust gas purifying catalyst in which a coating layer containing a catalytically active substance and a rare earth oxide is formed on a support. Is characterized by containing at least praseodymium oxide or terbium oxide.

[0008] Praseodymium oxide and terbium oxide are
It has an OSC like cerium oxide and has a high OSC per unit weight before and after performing a high-temperature durability test.
Have been confirmed by the present inventors to be significantly superior to cerium oxide and a composite oxide of zirconium oxide and cerium oxide. Therefore, if praseodymium oxide or terbium oxide is used instead of, or in combination with, cerium oxide or a composite oxide of zirconium oxide and cerium oxide, it can withstand higher temperatures than a conventional exhaust gas purification catalyst (coating layer). Performance can be improved. Therefore, if the coating layer contains praseodymium oxide or terbium oxide as in the exhaust gas purifying catalyst of the present invention,
High OSC can be maintained even after high-temperature durability. This means that, unlike conventional exhaust gas purification catalysts, high OSC can be exhibited at high temperatures without using a large amount of OSC components, and the exhaust resistance of the engine can be improved by reducing the thickness of the coating layer. Means.

The rare earth oxides include oxides of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. However, as described above, in the present invention, the coating layer contains at least one of praseodymium oxide and terbium oxide.

The above-mentioned catalytically active substances include ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir),
Platinum (Pt) and gold (Au),
Preferably, at least one selected from the group consisting of platinum, rhodium and palladium is selected.

As the above-mentioned support, cordierite,
A honeycomb carrier made of mullite, α-alumina, metal (for example, stainless steel), or the like and having a large number of cells formed thereon can be used. When a honeycomb carrier is used, a coating layer is formed on the inner surface of each cell.

The coating layer is usually made of alumina.
(Al_TwoO_Three), Silica (SiO_Two), Titania (Ti
O_Two), Magnesia (MgO) or zirconia
(ZrO _Two) And other inorganic oxides with high heat resistance.
More.

Further, the catalytically active substance may be selectively carried on a rare earth oxide or an inorganic oxide in advance, or may be later carried on the surface of a layer formed by these oxides.

In the exhaust gas purifying catalyst, the coating layer containing the catalytically active substance and the rare earth oxide may be formed as a single layer or as a plurality of layers. When the coating layer is composed of a plurality of layers, at least one of the layers only needs to contain praseodymium oxide or terbium oxide, and whether or not each layer contains praseodymium oxide or terbium oxide depends on the catalyst used. What is necessary is just to design suitably according to the kind of active substance, etc.

[0015] In a preferred embodiment, the coating layer contains palladium and rhodium as the catalytically active substances, and palladium and rhodium are contained in different layers.

Palladium has the advantage of being excellent in low-temperature activity, but has poor compatibility with rhodium. Therefore, if palladium is contained in a layer different from rhodium as in the above configuration, the effect of rhodium on palladium can be appropriately avoided.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to an exhaust gas purifying catalyst in which a coating layer containing a catalytically active substance and a rare earth oxide is formed on a supporting carrier, wherein at least praseodymium oxide or terbium oxide is contained as the rare earth oxide. The feature is that it is.

[0018] Praseodymium oxide and terbium oxide are
It has the same OSC as cerium oxide, and has a higher OCS per unit mass than cerium oxide and a composite oxide of cerium oxide and zirconium oxide before and after the durability treatment in a high-temperature atmosphere. This has been confirmed by the present inventors by the following method.

The present inventors have proposed praseodymium oxide (Pr).
₄ oxides of ₆ O ₁₁ ), terbium oxide (Tb ₄ O ₇ ), cerium oxide (CeO ₂ ), and a composite oxide of cerium oxide and zirconium oxide (Ce _0.8 Zr _0.2 O ₂ ) were prepared. The oxide was evaluated for OSC before and after the durability treatment. The result is shown in FIG. The OSC evaluation and the durability processing are as described below.

(Evaluation of OSC) Each of the above oxides was exposed to an oxidizing atmosphere containing oxygen for 40 minutes to occlude oxygen, and the weight was measured. Thereafter, each of the oxides storing oxygen was exposed to an inert atmosphere for 3 minutes, and further exposed to a hydrogen-rich reducing atmosphere for 7 minutes, and the weight was measured.
The ability of each oxide to release oxygen in a reducing atmosphere was evaluated by subtracting the weight after exposure to a reducing atmosphere from the weight after each exposure to an oxidizing atmosphere. In addition,
The amount of released oxygen was evaluated in terms of per gram of each composite oxide. In addition, each atmosphere is a gas having a composition shown in
at a flow rate of dm ³ / hr,
The ambient temperature is maintained at 500 ° C.

[0021]

[Table 1]

(Durability treatment) The durability treatment was performed in an oxidation-reduction atmosphere maintained at about 1000 ° C. Specifically, an inert atmosphere of 5 minutes, an oxidizing atmosphere of 10 minutes, an inert atmosphere of 5 minutes, and a reducing atmosphere of 10 minutes, which is a total of 30 minutes, is defined as one cycle. Was alternately exposed to an oxidizing atmosphere and a reducing atmosphere. In addition, each atmosphere is a gas having a composition shown in Table 2 containing high-temperature steam at 300 dm ³ /
It is achieved by supplying at a flow rate of hr, and the ambient temperature is maintained at about 1000 ° C. by high-temperature steam.

[0023]

[Table 2]

As is clear from FIG. 1, praseodymium oxide (Pr ₆ O ₁₁ ) and terbium oxide (Tb ₄ O ₇ )
Represents cerium oxide (CeO ₂ ) and its composite oxide (C
OSC is much higher than that of e _0.8 Zr _0.2 O ₂ ) before and after the durability treatment in a high-temperature atmosphere. Therefore, if at least praseodymium oxide or terbium oxide is contained in the coating layer as in the exhaust gas purifying catalyst of the present invention, high OSC can be maintained and exhibited not only before high-temperature durability but also after durability. Becomes possible.

Next, some specific embodiments of the exhaust gas purifying catalyst according to the present invention will be briefly described with reference to FIGS.

The exhaust gas purifying catalyst 1 shown in FIG. 2 has a structure in which a single coating layer 3 is formed on a cell inner surface 20 of a honeycomb support 2 made of cordierite or the like.
The coating layer 3 is composed of an inorganic oxide A having high heat resistance such as alumina, a rare earth oxide B containing praseodymium oxide or terbium oxide, and a catalytically active substance C such as platinum, palladium or rhodium. These substances A, B, and C coexist without any chemical bond as schematically shown in the drawing. Of course, the above-mentioned substances A, B,
Components other than C may be included.

The exhaust gas purifying catalyst 1 having such a configuration
Is prepared by mixing a salt solution containing an inorganic oxide A, a rare earth oxide B, and a catalytically active substance C, which has been pulverized and classified into a certain size by using a ball mill or the like to form a slurry, and the slurry is formed into a honeycomb. After the support carrier 2 is immersed, it is formed by pulling it up and performing a heat treatment.

Examples of the salt solution containing the catalytically active substance C include:
A nitrate aqueous solution, a dinitrodiammine nitrate aqueous solution, a chloride aqueous solution and the like are used.

The heat treatment is performed at about 50-200 ° C. for about 1-48.
After drying for about 1 hour at about 350-1000 ° C.
It is performed by firing for a time.

The exhaust gas purifying catalyst 1 shown in FIG. 3 has a cell inner surface 2 of a honeycomb support 2 similar to the previous embodiment.
0, a single coating layer 3 containing an inorganic oxide A, a rare earth oxide B, and a catalytically active substance C is formed. In the exhaust gas purifying catalyst 1 of FIG.
And a rare earth oxide B is first coated on the inner surface 20 of the cell, and a catalytically active substance C is supported on the surface of a layer composed of these substances A and B.

The exhaust gas purifying catalyst 1 having such a configuration
Is obtained as follows. First, the inorganic oxide A and the rare-earth oxide B are mixed, and distilled water is added thereto to form a slurry. After the honeycomb support 2 is immersed in the slurry, the honeycomb support 2 is pulled up and heat-treated, so that the inorganic oxide A
And a layer made of rare earth oxide B. Next, a solution of a salt containing the catalytically active substance C is prepared, and the solution is impregnated with the honeycomb support 2 that has been subjected to the above treatment, and then heat-treated again.
Is obtained.

The heat treatment after the honeycomb support 2 is immersed in the slurry and the heat treatment when the catalytically active substance C is carried are performed under the same conditions as the heat treatment of the above-described embodiment.

The exhaust gas purifying catalyst 1 shown in FIG. 4 has an inorganic oxide A, a rare earth oxide B and a catalytically active substance C on the inner surface 20 of the cell of the honeycomb support 2 as in the previous two embodiments. And a single covering layer 3 containing In the exhaust gas purifying catalyst 1 of FIG. 1, in a state where the catalytically active substance C is supported on both the inorganic oxide A and the rare earth oxide B, the cell inner surface 20 of the honeycomb support 2 is formed.
Is formed with a coating layer 3.

The exhaust gas purifying catalyst 1 having such a configuration
Is obtained as follows. First, the inorganic oxide A and the rare-earth oxide B are mixed, crushed and classified into a predetermined size by using a ball mill or the like, and this is mixed with a salt solution containing the catalytically active substance C to form a slurry. Next, the slurry is heat-treated to support the catalytically active substance C on each of the inorganic oxide A and the rare-earth oxide B. The resultant is again ground and classified by a ball mill, and then distilled water is added to form a slurry. After the honeycomb support 2 is immersed in the slurry, the honeycomb support 2 is pulled up and heat-treated, so that the coating layer 3 as shown in FIG. 3 is formed on the cell inner surface 20 of the honeycomb support 2.

The heat treatment that is appropriately performed is performed in the same manner as the heat treatment of the above-described embodiment.

Of course, the catalytically active substance C may be carried on one of these oxides A and B without carrying the catalytically active substance C on both the inorganic oxide A and the rare earth oxide B.

In the exhaust gas purifying catalyst 1 shown in FIG. 5, the coating layer 3 has a two-layer structure composed of a first coating layer 3A and a second coating layer 3B. The first coating layer 3A and the second coating layer contain inorganic oxides A and a, rare earth oxides B and b, and catalytically active substances C and c.
B) shows the coating layer 3 of the exhaust gas purifying catalyst 1 shown in FIG.
The configuration is the same as described above. That is, each coating layer 3A (3
In B), the catalytically active substance C (c) is not supported on any of the inorganic oxide A (a) and the rare earth oxide B (b), and these oxides A (a), B (b) ) And coexist.

The exhaust gas purifying catalyst 1 having such a configuration
After forming the first coating layer 3A in the same manner as the coating layer 3 of the exhaust gas purifying catalyst 1 shown in FIG. 2, a second coating layer is formed on the first coating layer 3B by the same method. Obtained by:

In such a configuration, the catalytically active substance C
When palladium is used as the material, it is preferable to include palladium in the first coating layer 3A on the inner layer side.

Further, since palladium and rhodium are incompatible with each other, when rhodium is used in combination with palladium as the catalytically active substances C and c, it is preferable that palladium and rhodium are respectively contained in the coating layers 3A and 3B different from palladium and rhodium. . More specifically, it is preferable to include palladium in the first coating layer 3A on the inner layer side and rhodium in the second coating layer 3B on the outer layer side.

Since palladium has a good compatibility with praseodymium oxide and platinum has a good compatibility with cerium oxide, when palladium and platinum are used in combination as the catalytically active substances C and c, the first coating layer 3A is formed. Praseodymium oxide is used as the rare earth oxide B, and the second coating layer 3
It is preferable to use cerium oxide as the rare earth oxide b of B.

Of course, the structure of the first coating layer 3A and the second coating layer 3B may be the same as the structure of the coating layer 3 of the exhaust gas purifying catalyst 1 shown in FIGS. 3 and 4. Alternatively, the coating layer 3 may have three or more layers.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of evaluating the OSC of various oxides.

FIG. 2 is an enlarged schematic view showing an overall configuration and an essential part of an embodiment of an exhaust gas purifying catalyst according to the present invention.

FIG. 3 is an enlarged schematic view of a main part of another embodiment of the exhaust gas purifying catalyst according to the present invention.

FIG. 4 is an enlarged schematic view of a main part showing still another embodiment of the exhaust gas purifying catalyst according to the present invention.

FIG. 5 is an enlarged schematic view of a main part showing still another embodiment of the exhaust gas purifying catalyst according to the present invention.

[Brief description of reference numerals]

 Reference Signs List 1 Exhaust gas purification catalyst 2 Honeycomb carrier (as support carrier) 20 Inner surface of cell 3 Coating layer 3A First coating layer 3B Second coating layer A, a inorganic oxide B, b rare earth oxide C, c catalytic activity material

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 4D048 AA06 AA13 AA18 AB01 AB02 AB05 BA01Y BA03X BA06Y BA07Y BA08X BA10X BA12X BA18X BA19X BA30X BA31X BA32Y BA33X BA34Y BA39Y BA41X BA42X BB02 BB16 BA01A03 BA03A01 BA03A03 BA18 BB02A BB02B BB04A BB04B BB06A BB06B BC32A BC33A BC38A BC39A BC40A BC41A BC42A BC43A BC43B BC44A BC44B BC51B BC70A BC71A BC71B BC72A BC72B BC73A BC74A BC75A BC75B CA03 CA09 EA18 EC22 FA03 EA18 EC06

Claims (5)

[Claims] 1. An exhaust gas purifying catalyst having a coating layer containing a catalytically active substance and a rare earth oxide formed on a supporting carrier, wherein the rare earth oxide contains at least praseodymium oxide or terbium oxide. An exhaust gas purifying catalyst, characterized in that: 2. The exhaust gas purifying catalyst according to claim 1, wherein the catalytically active substance is at least one selected from the group consisting of platinum, rhodium, and palladium. 3. The exhaust gas purifying catalyst according to claim 1, wherein the coating layer comprises a plurality of layers, and at least one of the layers contains praseodymium oxide or terbium oxide. 4. The exhaust gas purifying catalyst according to claim 3, wherein the coating layer contains palladium and rhodium as the catalytically active substances, and palladium and rhodium are contained in different layers, respectively. 5. The exhaust gas purifying catalyst according to claim 3, wherein the coating layer contains palladium as the catalytically active substance, and the palladium is contained in other than the outermost layer.